Why Does Earth's Magnetic Field Flip?

The model is essentially a set of equations that describe the physics of the geodynamo. The equations are continually solved, each solution advancing the clock forward about a week. At its longest stretch, the model ran the equivalent of 500,000 years, Glatzmaier said.

By studying the model, the scientists discovered that, as the geodynamo generates new magnetic fields, the new fields usually line up in the direction of the existing magnetic field.

"But once in a while a disturbance will twist the magnetic field in a different direction and induce a little bit of a pole reversal," Glatzmaier said.

These bits of a pole reversal are referred to as instabilities. They constantly occur in the fluid flow of the core, tracking through it like little hurricanes, though at a much slower paceabout one degree of latitude per year.

Typically, instabilities are temporary. But on very rare occasions, conditions are favorable enough that the reversed polarity gets bigger and bigger as the original polarity decays. If this new polarity takes over the entire core, it causes a pole reversal.

"It's a very complicated, chaotic system, and it has a life of its own," Glatzmaier said.

Weak Spot

Peter Olson, a geophysicist at Johns Hopkins University in Baltimore, Maryland, said scientists can now pinpoint the core-mantle boundary where these instabilities in the magnetic field are happening.

One such disturbance Olson has been observing recently formed over the east-central Atlantic Ocean. Like a little hurricane, the anomaly swept toward the Caribbean and is moving up in the direction of North America.

"It's a new one, a little thing," Olson said. "Time will tell whether it develops into something significant. But it is here in the North Atlantic, moving towards the Pentagon. We can track it over the next couple of decades."

Instabilities such as this, Olson added, are causing Earth's magnetic field to weaken. Today the field is about 10 percent weaker than it was when German mathematician Carl Friedrich Gauss first began measuring it in 1845. Some scientists speculate the field is headed for a reversal.